CVE-2026-23111netfilter: nf_tables: fix inverted genmask check in nft_map_catchall_activate()

Official description

In the Linux kernel, the following vulnerability has been resolved: netfilter: nf_tables: fix inverted genmask check in nft_map_catchall_activate() nft_map_catchall_activate() has an inverted element activity check compared to its non-catchall counterpart nft_mapelem_activate() and compared to what is logically required. nft_map_catchall_activate() is called from the abort path to re-activate catchall map elements that were deactivated during a failed transaction. It should skip elements that are already active (they don't need re-activation) and process elements that are inactive (they need to be restored). Instead, the current code does the opposite: it skips inactive elements and processes active ones. Compare the non-catchall activate callback, which is correct: nft_mapelem_activate(): if (nft_set_elem_active(ext, iter->genmask)) return 0; /* skip active, process inactive */ With the buggy catchall version: nft_map_catchall_activate(): if (!nft_set_elem_active(ext, genmask)) continue; /* skip inactive, process active */ The consequence is that when a DELSET operation is aborted, nft_setelem_data_activate() is never called for the catchall element. For NFT_GOTO verdict elements, this means nft_data_hold() is never called to restore the chain->use reference count. Each abort cycle permanently decrements chain->use. Once chain->use reaches zero, DELCHAIN succeeds and frees the chain while catchall verdict elements still reference it, resulting in a use-after-free. This is exploitable for local privilege escalation from an unprivileged user via user namespaces + nftables on distributions that enable CONFIG_USER_NS and CONFIG_NF_TABLES. Fix by removing the negation so the check matches nft_mapelem_activate(): skip active elements, process inactive ones.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/8c760ba4e36c750379d13569f23f5a6e185333f5
• https://git.kernel.org/stable/c/b9b6573421de51829f7ec1cce76d85f5f6fbbd7f
• https://git.kernel.org/stable/c/42c574c1504aa089a0a142e4c13859327570473d
• https://git.kernel.org/stable/c/1444ff890b4653add12f734ffeffc173d42862dd
• https://git.kernel.org/stable/c/8b68a45f9722f2babe9e7bad00aa74638addf081
• https://git.kernel.org/stable/c/f41c5d151078c5348271ffaf8e7410d96f2d82f8
• https://cert-portal.siemens.com/productcert/html/ssa-082556.html
• https://cert-portal.siemens.com/productcert/html/ssa-253495.html
• https://cert-portal.siemens.com/productcert/html/ssa-019113.html
• https://cert-portal.siemens.com/productcert/html/ssa-457936.html
• https://blog.exodusintel.com/2026/06/08/off-by-exploiting-a-use-after-free-in-the-linux-kernel/ technical-description
• https://access.redhat.com/security/cve/CVE-2026-23111 vdb-entry, x_refsource_REDHAT
• RHBZ#2439687 issue-tracking, x_refsource_REDHAT
• https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-23111.json x_sadp-csaf-vex
• https://access.redhat.com/errata/RHSA-2026:10996 vendor-advisory, x_refsource_REDHAT
• https://access.redhat.com/errata/RHSA-2026:18134 vendor-advisory, x_refsource_REDHAT
• https://access.redhat.com/errata/RHSA-2026:10108 vendor-advisory, x_refsource_REDHAT
• https://access.redhat.com/errata/RHSA-2026:9112 vendor-advisory, x_refsource_REDHAT
• https://access.redhat.com/errata/RHSA-2026:6570 vendor-advisory, x_refsource_REDHAT
• https://access.redhat.com/errata/RHSA-2026:62639 vendor-advisory, x_refsource_REDHAT
• https://access.redhat.com/errata/RHSA-2026:62641 vendor-advisory, x_refsource_REDHAT